Considerable effort is directed to efficiently and effectively harnessing solar energy. Photo voltaic (PV) panels convert received solar rays directly into electrical energy, while thermal systems concentrate solar rays to produce heat, which can be used directly for heating purposes, or is used to produce rotating mechanical power by operating a Stirling engine for example, or more commonly by creating steam to operate a steam turbine, which can then be used to produce electrical power. Solar power may also be concentrated on a PV panel to increase the amount of solar energy received on a given area of panel, and thus increase the power generated from the panel.
A typical solar collection and concentration system uses mirrors to reflect solar rays from a wide area onto a much smaller target such that the solar energy from the wide area is concentrated at the target. In concentrating point focus systems a parabolic dish focuses sunlight onto a solar receiver. The parabolic shape is preferred as it focuses the solar rays on a small target, thereby increasing the temperature at the target. Dishes can typically vary in size and configuration from a small diameter of perhaps 1 meter to much larger structures of a dozen or more meters in diameter. Point focus dish concentrators are mounted on tracking systems that track the sun in two axes, directly pointing at the sun, and the receiver is attached to the dish at the focal point so that as the dish moves, the receiver moves with it. These point focus systems can generate high temperatures exceeding 800° C. and even 1,800° C.
In central tower systems a solar receiver is mounted at the top of a tower. The tower is surround by heliostats, which are flat mirrors that redirect sunlight to the top of the tower and are controlled to track the sun and move so that sunlight is reflected in a constant direction at the receiver. Hundreds or thousands of heliostats may work in unison to direct sunlight to the top of the tower to generate high temperatures from 500° C. to 1,100° C. or more, typically used to operate a steam turbine.
A trough concentrator is a long concave trough shaped concentrator that forms a line focus instead of a point focus as is achieved in both the Point Focus Dish and Central tower designs. Again a parabolic shape is preferred for the cross section of the trough to concentrate the rays on a narrow line. This has typically been the lowest cost solar collection technology due to its simplicity. The long trough concentrator focuses its light onto a solar receiver tube that runs the length of the solar trough concentrator. Depending on the latitude of the location, the troughs are oriented length wise in a North-South or East-West direction, and then pivot on their longitudinal axes such that the concave faces of the troughs follow the sun. These trough concentrators thus are only required to pivot on one axis, rather than on two axes as is required with point focus concentrators. Trough concentrators cannot produce as high a temperature as either the point focus dish or central tower designs since it cannot focus to a point and can only focus to a line. Temperatures of typically less than 600 C can be achieved.
A major cost of any operation using solar energy is in the solar collection field, which can be made up of hundreds or thousands of concentrators. The reflective surface is typically provided by polished aluminum or glass mirrors. The glass or metal sheets of the mirror can be curved to form the required focusing shape or are often made up of an array of smaller flat mirror segments arranged on a frame to achieve the focusing effect. The mirrors are heavy, requiring significant structural support to carry the load of the mirrors and provide sufficient rigidity to maintain focus. These heavy structures also require heavy and expensive motors and gear boxes to steer the solar array to appropriately focus the sunlight.
To reduce the demands on the frame and tracking system light weight linear tensioned thin-film trough concentrators have been developed where a thin film of reflective sheet material is stretched over a framework. The framework comprises parabolic shaped ribs mounted side by side along the length of the trough, and then the film is attached to one end of the trough and stretched along the outside convex faces of the ribs and attached under tension at the opposite end. Such thin film trough concentrators are disclosed for example in United States Patent Application Number 2010/0258186 of Harrienstien et al. and U.S. Pat. No. 8,056,555 to Prueitt.
These thin film trough concentrators significantly reduce the weight of the mirror apparatus compared to more traditional designs utilizing rigid backed reflective material, with a corresponding reduction in the strength requirements of the frame and tracking system. The framework must however be sufficiently strong to resist deformation due to the considerable tension on the film stretched from end to end of the trough.